Our goal is to analyze the interactions between the sensory end-organ and the CNS in the early development of a sensory system and to gain some insight into the mechanisms responsible for the formation of sensory neurons and their synaptic connections. The primary and secondary sensory neurons of the auditory system are being studied with light and EM methods in chick embryos to define the structural relationships between the cochlear nerve fibers, their synaptic endings, and the hair cell receptors of the cochlear duct, on the one hand, and the second-order neurons of the cochlear nucleus (nucleus magnocellularis), on the other hand, at each stage in the development of the auditory pathways. We determine the intermediate stages in the differentiation of specific types of neurons by light and electron microscopy and use experimental methods to discover what factors influence their development and at what particular intermediate developmental stage and anatomical locus. We propose to define the role of tissue interactions in differentiation by use of end-organ ablation, tissue culture and transplantation experiments.